Generally, a capacitive microphone utilizes a capacitance between a pair of electrodes which are facing each other to generate an acoustic signal. A MEMS microphone may be manufactured by a semiconductor MEMS process to have an ultra-small size.
A MEMS microphone may include a substrate including a cavity, a bendable diaphragm and a back plate which is facing the diaphragm. The diaphragm can be a membrane structure to generate a displacement due to the acoustic pressure. In particular, when the acoustic pressure reaches to the diaphragm, the diaphragm may be bent upwardly or downwardly due to the acoustic pressure. The displacement of the diaphragm can be sensed through a change of capacitance between the diaphragm and the back plate. As a result, an acoustic wave can be converted into an electrical signal for output.
The characteristics of a MEMS microphone can be determined by measuring various factors such as: a pull-in voltage, sensitivity, a frequency resonance, and a total harmonic distortion (THD). In particular, a THD value may serve as an indicator for or symptom of unnecessary harmonic components in the acoustic signal to result in distortion. The distortion component is a component tone which is not originally included in the input acoustic signal, which may cause sound quality to degrade upon reproduction. Therefore, a MEMS microphone should ideally be configured such that the THD value does not exceed a proper level. It can be especially important to maintain an appropriate level of the THD value in cases where the flexibility of the diaphragm is low.